Field of Invention
The invention relates to a load-handling means for a tower or a tower section of a wind turbine, which load-handling means has tower-attachment means for attachment to an upper end or in the region of an upper end of a tower or a tower section of a wind turbine, and attachment points for attaching at least one anchoring means of a lifting gear unit. The invention also relates to a method for erecting a wind turbine, in particular an offshore wind turbine.
Brief Description of Related Art
Modern wind turbines comprise a high tower with a vertical longitudinal axis and frequently composed of a plurality of tower sections, at the tip of which tower a nacelle or a gondola with a rotor with a plurality of rotor blades and a horizontal rotor axis is mounted so as to be rotatable about the tower axis.
Towers of onshore wind turbines are usually erected section-wise from a plurality of sections and erected directly in situ at the construction site on a base. Tower sections for such wind turbines are usually transported in a horizontal position to the erection site. When a tower of a wind turbine is erected, the, generally multiple, individual tower sections are successively moved from their horizontal position into an upright position using a lifting gear unit and fitted onto a base, an auxiliary base or a clamping point and/or a tower section which was the last to be erected, and are connected thereto. In the process, the longitudinal axis of the tower is moved from a horizontal orientation into a vertical orientation.
In contrast, towers for offshore wind turbines are preferably erected section-wise on auxiliary bases at the port from where they are shipped. Subsequently, the towers which are completely erected, or the tower sections which are placed in a vertical position, for transportation are loaded at the port onto a wind turbine installation vessel and attached there to beam-grillage-like auxiliary bases for transportation, referred to as “grillages”. For reasons of space, offshore towers are preferably transported in an upright position. The nacelle, referred to as a gondola, is not attached to the tower until after the tower has been completely erected.
Wind turbine towers are components which are very susceptible to oscillations. However, a freestanding tower without a gondola has a significantly different natural frequency than a tower with a nacelle and rotor arranged at the tip.
This means that during the erection of a wind turbine a freestanding tower without a gondola, or a tower shell, and is not yet loaded at its tip with the weight of the nacelle oscillates at a higher frequency than after the nacelle is fitted on.
Any structure, and therefore also a wind turbine or a wind turbine tower, reacts to external excitation, for example by wind or waves, at a specific frequency with natural oscillations, depending on the frequency of the excitation. In addition, any structure has what are referred to as natural frequencies. These are the frequencies at which the system oscillates if it is deflected and then left to its own devices. For wind turbines, in particular the first natural frequency is relevant, the associated first eigen mode of which is composed essentially of flexural deformation of the tower. It is therefore also referred to as the “first flexural natural frequency”. The associated eigen mode is the “first flexural eigen mode”. This frequency changes when further components are added, for example the wind turbine tower becomes taller. It is therefore dependent to a considerable extent on the state of construction.
Within the scope of the present invention, the term natural frequency comprises in the present context the natural frequency of a wind turbine tower per se and the natural frequency of a combination of the tower and (auxiliary) base and clamping means, that is to say essentially the natural frequency of the tower which is clamped or attached on one side.
Oscillation excitation becomes critical particularly when it is superimposed in the frequency range with the natural frequency of the structure and thereby brings about resonant oscillations which can destroy the structure.
Wind turbines are mainly excited to oscillate by the impinging wind. In completely erected turbines which are put into operation, oscillations at the tower occur in particular in reaction to the loading of the rotor by the wind. In the case of offshore wind turbines, the excitation from the movement of the sea in the form of impinging waves must also be taken into account.
The main causes of the oscillations of freestanding turbine shells are vortices, what are referred to as Karman's vortices, which bring about vortex-induced lateral oscillations (“vortex-induced vibrations”—VIV). This involves a series of vortices shared by the wind or wind field flowing around the tower alternately on the left-hand side and right-hand side of the tower viewed in the direction of the wind. The oscillation of the tower and the vortices have a mutually reinforcing effect. After the nacelle has been fitted on and the wind turbine has been put into operation, the natural frequency of the tower changes in such a way that vortex-induced vibrations are superimposed with the natural frequency of the tower at only low wind speeds and are then less critical.
Basically, the natural frequencies of the towers without a gondola are higher than in the case of completely assembled turbines. There are then conditions under which excitations by Karman's vortices induce VIVs at frequently occurring wind speeds. Correspondingly, the amplitudes of the oscillations become larger, with the result that the mechanical and structural loads acting on the wind turbine towers increase correspondingly.
Since the VIVs occur at certain wind speeds, towers of wind turbines in the shell condition without a nacelle fitted on either have to be secured above a certain length, for example by anchoring means, or measures have to be taken to prevent vortices. Otherwise, VIVs can cause the tower to be destroyed. Below a critical length, this risk does not occur, or only does so to a significantly smaller extent, since the natural frequencies of the relatively short tower stumps with a smaller number of tower sections lie in a relatively high frequency range, and the high wind speeds which are associated with said frequencies do not have to be anticipated in reality.
The critical wind speed at which vortex-induced lateral oscillations are dangerous for the tower shell, is usually between 10 m/s and 25 m/s for a completely erected tower without a gondola, depending on the height and rigidity of the tower, for which reason erection is not permitted if these wind speeds are expected and the gondola cannot be pulled up directly after the lifting of the last tower length.
When erecting a wind turbine, this fact is allowed for in that, during the erection of a wind turbine with a tower composed of a plurality of sections the last section or, if appropriate, the last sections of the tower and of the nacelle are installed or fitted onto the tower in a weather window with calm weather, preferably at wind speeds of at maximum 9 m/s. Such weather windows must last for at least several hours or days. Therefore, waiting for a suitable weather window can considerably delay the erection of a wind turbine, and as a result incur high costs.
The consequences of the oscillation problem are more pronounced offshore than onshore. In contrast to the section-wise erection of onshore towers, offshore towers are, as a rule, already assembled at the port from a plurality of tower sections with a vertically positioned longitudinal axis, and for reasons of space are preferably transported in an upright position on a wind turbine installation vessel. During transportation, the towers or tower sections are subjected to excitation by wind and waves. VIVs can already occur at this point. Furthermore, the weather windows which are suitable for erection are shorter offshore owing to generally more continuous and higher wind speeds, with the result that, as a rule, it is necessary to wait longer for a weather window which is suitable for erection than onshore, which is in turn associated with correspondingly high additional costs for the offshore construction site.
During transportation on a wind turbine installation vessel, there is also the factor that the dynamic wind can additionally amplify the wind which is present in any case, such that even stronger VIVs can occur. Furthermore, a tower or a tower section can additionally be excited to oscillate by rolling movements of the vessel which are caused by the swell of the sea. Wind turbine installation vessels are usually not equipped with their own liquid rolling movement damper which would damp these rolling movements, since the loading with the vertical towers or tower sections would shift the rolling frequency in such a way that such a damper can have little effect. The rolling movement also brings about a certain amount of air flow at the tips of the towers or tower sections.
At present, the towers of the offshore wind turbines are, as a rule, transported in two pre-assembled sections which are subsequently fitted successively onto the foundation structure or onto the lower tower section in two lifting operations and connected. As a result of the divided transportation of the tower, the critical wind speeds for excitation by VIVs are so high for the lower section that they do not occur to a critical extent.
However, if complete towers are to be installed in order to shorten the time for offshore work, the critical wind speeds owing to the increased tower height are then below 20 m/s and can therefore realistically occur. In this case, measures to counteract VIVs must be urgently taken.
Since towers for offshore wind turbines are, as a rule, pre-installed at the port to form an upright tower shell and/or transported in an upright position on an installation vessel, owing to VIVs above a certain length they have hitherto either been secured by bracing means, or coils are attached in the upper tower region in order to prevent vortices, such as is known, for example, in WO 2006/106162 A2. However, the practicable use of coils requires compliance with specific criteria relating to the thickness of the coils and the gradient of the winding. The expenditure on the installation and de-installation of these coils offshore is considerable.